This invention relates to refrigeration systems, and in particular, to a method and apparatus for obtaining the maximum efficiency from a refrigeration system. While the invention is described in detail with respect to conventional air conditioning systems, those skilled in the art will recognize the wider applicability of the invention disclosed hereinafter. The invention may find additional use, for example, with heat pumps, refrigeration systems, or other applications where system efficiency may be improved by monitoring specific parameters affecting that efficiency.
The operational components of a conventional air conditioning system are well known in the art. In general, such systems include a compressor which forces the particular refrigerant used in the system through a condensing coil, and then through an expansion valve into an evaporator coil. The refrigerant is sent back to the suction side of the compressor from the evaporator coil, after which the cycle is repeated. The expansion valve plays an important part of the overall efficiency exhibited by the system. Under ideal operating conditions, the expansion valve should admit an amount of refrigerant that can be evaporated or slightly superheated in the evaporator coil. That is to say, the evaporator coil should be wetted along approximately its entire length to provide good heat transfer rate and maximum refrigeration system efficiency. In the past, some portion of the evaporator coil always has been dry. A dry evaporator coil portion was utilized in order to prevent the passage of liquid to the suction side of the compressor. Liquid entering the suction side of the compressor causes damage to the compressor valves. Consequently, it is the prevalent practice to design refrigeration systems with a safety margin so that the coil is operating at its most efficient point at light load conditions. That is to say, with light load conditions, the maximum coil length is available for heat transfer. However, as load increases, the length of the coil available for effective heat transfer decreases so that heavy load conditions represent the least efficient operating area of the refrigeration system.
Thermostatic control valves presently are the most prevalent means for controlling the operation of refrigeration systems. Thermostatic control valves generally include a diaphragm actuated valve member having one side of the diaphragm operatively connected to a pressure generating means. The pressure generating means conventionally is a sealed sensor having a gas responsive to temperature enclosed in it. The opposite side of the diaphragm is opposed by system pressure and the diaphragm is preloaded by means of a spring to set the operating point of the valve. Pressure changes in the gas of the sensor, in response to changes in temperature, operate the valve. While these devices work well for their intended purposes, the thermostatic expansion valve can not adequately improve system efficiency by assuring full utilization of the refrigeration coil, because a comparatively small system gain must be used to maintain system stability.
Thermostatic expansion valves also suffer an additional disadvantage in heat pump applications. As will be appreciated, a heat pump, for explanational purposes, may be considered a reverse cycle refrigeration system. Consequently, two thermostatic expansion valves must be used, since a different adjustment of the valves normally is required for each coil. Thermostatic expansion valves also generally control flow only in one direction through the valve and since heat pumps generally reverse fluid flow through the evaporator and condenser coils, an additional valve is required in heat pump applications. Such duplication results in increased cost.
A number of devices are known in the art which function to improve the efficiency of air conditioning or heat pump systems. One particular advantageous system is shown and disclosed in the co-pending application by Behr, Ser. No. 720,698, now U.S. Pat. No. 4,067,203 ('203). In '203, a single function of the system is monitored, and that system variable is used to control system operation. Another method of controlling an evaporator system is shown in a co-pending application by Behr, Ser. No. 862,446, filed Dec. 20, 1977, and assigned to the assignee of the present invention. The last mentioned Behr application monitors pressure and temperature of the refrigerant used in the refrigeration system to control position of the expansion valve.
Each of the related inventions discussed above are intended to increase the efficiency of refrigeration systems. However, each operates under different theories of operation and are structurally distinct from one another.
It has been recognized for many years in the refrigeration industry that the flow control valve or expansion device should feed fluid to the evaporator coil at such a rate that a nominal superheat is maintained at the evaporator outlet. This, in turn, results in a superheat level being maintained at the compressor suction. Consequently, the compressor is protected from damage caused either by overfeeding (flooding) or under feeding (starving) refrigerant fluid conditions. It also is known that compressor sump temperature is related to the flow rate and superheat level of the refrigerant. I have found that compressor sump temperature is very sensitive and fast to respond either to flooding or to starving refrigerant fluid conditions. Compressor manufacturers desire to maintain the sump temperature between safe upper and lower temperature limits to ensure proper operation of the compressor. However, due to the nature of mechanical expansion valves of the type described above, which attempt to hold a constant superheat, the sump temperature rises and falls with ambient temperature on the compressor and the evaporator coil, the evaporator coil being the outdoor coil in the heating mode of heat pump systems.
The invention disclosed hereinafter provides excellent steady state and transient control of superheat at the suction side of a compressor, which control is maintained by using the combination of a sump temperature electrical signal modified by an outdoor air temperature electrical signal. For example, a system superheat setting of 2 degrees to 5 degrees F. at the suction side of a compressor can be held over all operating conditions of the refrigeration system by means of the invention disclosed hereinafter, thus guaranteeing a full wetted evaporator coil, high system operating capacity and high efficiency operation.
One of the objects of this invention is to provide an improved means for controlling the refrigeration cycle of a refrigeration system.
Another object of this invention is to provide a method for operating the refrigeration system in which superheat of the system is monitored through sump temperature of the compressor.
Another object of this invention is to provide a refrigeration system which permits the operation of the evaporator coil in a wetted state of operation.
Other objects of this invention will be apparent to those skilled in the art in light of the following description and accompanying drawings.